1. Field of the Invention
The present invention relates to the field of television camera design, and in particular to the use of digital circuitry to perform amplitude transfer characteristic (gamma) correction.
2. Description of the Prior Art
One step in the alignment of a television camera is to perform proper gamma correction. Gamma correction is intended to create visual color match, under conditions of equal color temperature and luminance, between the original scene and its reproduction on a color picture tube. Since the phosphors in a conventional picture tube do not respond linearly to different voltage levels, gamma correction is performed by applying a non-linear transfer function to the video signal prior to transmission or recording. These transfer functions in cameras are typically used not only to implement gamma correction, but also to establish black level and white level which are independent of gamma correction. The usual form for this transfer function is shown in equation (1). EQU V=C * X.sup.gamma (1)
where:
V=Output voltage after gamma correction; PA1 X=Input light level such that Black level.ltoreq.X.ltoreq.White level; PA1 gamma=An exponent, typically about 0.45; and PA1 C=A constant
Digital gamma correctors have been built which operate in accordance with equation (1). While this approach is mathematically correct, it tends to produce quantization artifacts, particularly near the black level. A transfer function which follows equation (1) may have a gain near the black level of approximately 20. For a typical digital gamma corrector, the input signal quantization levels are evenly spaced in the range between the black level and the white level. A small number of input quantization levels near the black level correspond to a large range of output values. Consequently, comparatively small changes in incoming light level may result in relatively large steps in the gamma corrected signal. This results in sampling artifacts which may enhance signal noise causing dark areas of the reproduced image to appear speckled or to have exaggerated textures.
Gamma correction in video cameras generally deviates from equation (1) to provide greater detail in low-light areas, as noted in a textbook by Benson et al. entitled "HDTV Advanced Television for the 1990s", McGraw Hill Publishing CO., (1991), at page 10.4. For example, the Society of Motion Picture and Television Engineers SMPTE 240M standard for Television Signal Parameters 1125/60 specifies the following transfer characteristic for use in 1125/60 high definition television systems: EQU V=1.1115 X.sup.0.45 -0.1115.times..ltoreq.0.0228 (2b) EQU V=4.0* X X&gt;0.0228 (2b)
Benson et al., at page 10.10, discuss a method of dynamic range extension to enhance detail at the edge of the signal range. In particular, Benson et al. discuss the importance of capturing detail beyond normal peak white. One known method is by means of highlight-compression circuits. Rather than sharply clipping all signals above reference white, gain-reduction electronic circuits operating on levels several times the reference white level compress the higher level signals so that they fall within the range. The circuits employ a piecewise linear transfer function in which the gain is substantially lower above a knee.
In order to perform well with widely varying light levels, it is desirable to be able to independently adjust the black level and white level. As the gamma value increases, however, the slope of the transfer function for input signal values near the black level also increases. Increased gain in the gamma curve tends to increase the visibility of noise in the reproduced image.